Judges have consistently thrown out such measures as unconstitutional. In Feburary, a California appeals court struck down such a law. It ruled that targeting violent or sexual video games, while not expanding the law to cover R-rated movies or suggestive books, unfairly singled out the free speech rights of a particular industry.

Both the ratings recommend that the media deemed Mature or Restricted is for people ages 17 and older. An R-rated film may contain "adult themes, adult activity, hard language, intense or persistent violence, sexually-oriented nudity, drug abuse or other elements." On the other hand, titles given an M rating may have content that includes "intense violence, blood and gore, sexual content and/or strong language."

However, given the nudity aspect, Mature-rated games may actually be less explicit than R-rated movies. Note that the M rating doesn't include nudity – that's reserved for Adults Only titles, according to the ESRB:

Titles rated AO (Adults Only) have content that should only be played by persons 18 years and older. Titles in this category may include prolonged scenes of intense violence and/or graphic sexual content and nudity.

One could argue that there are plenty of R-rated movies with "prolonged scenes of intense violence and/or graphic sexual content and nudity."

Unless the Chicago Transit Authority has some legitimate reason for treating games differently than films, they're probably on the losing side of this fight.